An automated manual transmission, also known as an automatic mechanical transmission or semi-automatic transmission, is a system that employs electronic sensors, pneumatics, actuators and processors to execute gear shifts either under command of an operator or by a computer. Essentially it can be described as a robot operating a manual transmission. To properly shift gears the engine must provide precise and repeatable amounts of torque.
Any transmission that requires the operator to manually synchronize engine crank-shaft revolutions (RPM) with drive-shaft revolutions is non-synchronous. Although automobiles and light duty trucks are almost universally using synchronized transmissions, heavy duty trucks and machinery are still using non-synchronous manual transmissions for a number of reasons. The friction material, such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged steel. The simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a synchronized transmission is slower than that of shifting a non-synchronous transmission which over time has an economical impact as mileage can be reduced. However, there is a great deal of driver skill involved in changing gears. Non-synchronous transmissions are engineered with the understanding that a trained operator will be shifting gears in a known coordination of timing.
Heavy duty truck operators use a technique known as double clutching to change gears. The technique comprises the following steps to upshift. The operator releases the accelerator, depresses the clutch pedal so that the clutch opens, shifts the gearbox into neutral and then releases the clutch pedal so that the clutch closes. The operator waits for the engine speed to decrease to a level suitable for shifting into the next gear, at which point the operator opens the clutch again, shifts into and engages the next gear, closes the clutch, and finally applies the accelerator. An experienced operator can execute the whole maneuver efficiently, and the result is a very smooth gear change.
Conversely, in order to downshift, engine speed must be increased while the gearbox is in neutral before the next gear is engaged. The sequence of steps is as follows. The operator releases the accelerator, depresses the clutch pedal to open the clutch, pulls to neutral, releases the clutch pedal so that the clutch closes and applies the accelerator until the engine speed increases to synchronous speed before initiating gear engagement. When engine speed is synchronous with transmission speed, the driver opens the clutch, engages the gear, closes the clutch and applies the accelerator. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle (transmission) and engine accurately.
Keeping the clutch open while in neutral, as is performed during a typical shift in a synchronous transmission, gives more economy of driver motion and effort compared to double clutching. However, significant wear can take place on the separated clutch plates any time the engine and transmission have varying drive loads. In simple terms, wear occurs the more the clutch has to “slip” to match revolutions between the engine and transmission. Double clutching can minimize this clutch plate wear by encouraging matching of engine and transmission RPMs before the clutch is closed.
Disconnecting drive components during a gear shift by using a clutch properly unloads the engine and transmission of undue pressure applied by opposing components. Double clutching, although time consuming, eases gear selection when an extended delay or variance exists between engine and transmission speeds. Double clutching is typically a testing requirement when obtaining a commercial driver's license.
Experienced drivers use a technique known as clutchless shifting instead of double clutching when shifting gears. Heavy duty trucks carrying heavy loads, for example above 40 tons, can have up to 24 gears. Bringing a truck from standstill to full speed utilizing the double clutch technique requires a lot of effort and concentration from the driver. Many experienced drivers have learned when it is possible to shift gears without using the clutch. This technique is known as clutchless-shift, float-shift, or skip-shift, which eliminates the use of the clutch except when launching or coming to rest. However, uncoordinated execution of the clutchless-shift technique results in gears not synchronizing properly as they are engaged leading to an accelerated wear of the transmission. Both double clutching and clutchless-shift gear selection in non-synchronous manual transmissions requires a skilled driver in order to minimize wear on components and provide an optimal fuel economy.
There is an advantage in providing an automated manual transmission in a heavy duty truck or machinery as it relaxes the requirement for a skilled operator. More junior drivers can be employed to operate the equipment without increase wear on components and a reduction in fuel economy. However, there are a number of challenges in combining an automated manual transmission with an engine fuelled from a gaseous fuel, such as liquefied natural gas (LNG) or compressed natural gas (CNG).
In some engines fuelled with a gaseous fuel such as natural gas, the fuel is in a gaseous phase in a common fuel rail under pressure prior to entering the fuel injectors. A high pressure pump or compressor is used to increase the pressure of the gaseous fuel to a suitable injection pressure in the common fuel rail. In contrast to incompressible fuels such as diesel or gasoline, where it is relatively easy to achieve and maintain a high pressure, the high pressure pump or compressor in a gaseous fuel system is actively working to a greater degree to maintain the gaseous fuel pressure as gaseous fuel is injected into combustion chambers. The gaseous fuel injection pressure must be sufficient to both overcome an in-cylinder pressure experienced when the fuel injectors actuate and to inject the desired amount of fuel in the available time.
The no-load torque reporting accuracy of an engine, that is when the engine is not loaded by the transmission, must be well defined so that gears can be synchronized in the automated manual transmission during gear shifting events. No-load torque control is required whether the double clutch or the clutchless shift technique of gear shifting is employed. The engine must respond in a predictable manner when the gears engage and mesh and the load is transferred to the engine to provide a smooth transition instead of an abrupt and choppy shift event. The automated manual transmission expects that engine speed will remain constant if it commands zero net torque (indicated torque equal to the friction torque). The automated manual transmission is also counting on certain engine speed responses to small torque requests above existing friction torque to provide “sync torque” and “torque bumps”, both of which are used during automated shifting. Torque accuracy ultimately depends on fuel delivery accuracy.
The performance of fuel injectors can be characterized by relating a quantity of fuel delivered to their on-time (time open), for example see the plot in FIG. 1 illustrating a Fuel-On-time characteristic. The x-axis shows the commanded amount of fuel, and the y-axis shows the on-time required to deliver that amount of fuel. Each fuel injector has its own Fuel-On-time characteristic. When fuel injectors are manufactured, they are made to comply within a level of Fuel-On-time tolerance, but because of variability introduced by manufacturing tolerances, fuel injectors that are made within specifications are not identical. Additionally, there is significant fuel injection flow change through the fuel injector break-in period. For example, after the first 30 to 50 hours of injection in an engine there can be significant changes in fuel flow through the injectors. Fuel delivery through the injectors continues to change over the operating life of the engine, although more slowly than during the initial break-in period. There can be many reasons for such changes including gas-hole carboning which is caused by the formation of carbon deposits. In dual fuel injectors which inject both a pilot fuel and a primary fuel, such as diesel and natural gas respectively, it has been found that injections for both fuels exhibit the above mentioned behavior.
With reference to FIG. 1, it has been found that the portion of the plot above a critical on-time TC is within an acceptable level of tolerance for the torque reporting accuracy requirements of the automated manual transmission from fuel injector to fuel injector across production runs and throughout the operational life cycle. In contrast, the portion of the plot below the critical on-time TC has been found not to be within an acceptable level of tolerance from fuel injector to fuel injector within production runs and throughout the operational life cycle. Accordingly an observed problem is a variation from fuel injector to fuel injector in the amount of fuel injected into a combustion chamber for a commanded short on-time (small fuellings) under consistent differential pressure between fuel rail pressure and in-cylinder pressure. Referring to FIG. 2, a scatter diagram illustrating torque reporting accuracy for uncalibrated fuel injectors is shown. The x-axis shows the commanded net engine torque and the y-axis shows the measured mean brake torque based on a commanded quantity of fuel. An upper torque threshold line 10 and a lower torque threshold line 15 show the range of allowable measured torques for each commanded torque for the automated manual transmission. At lower torques the allowed difference between the upper and lower threshold is less than at higher torques. Data is shown plotted for four different sets of injectors. At torques below approximately 1250 Nm, the measured torque for some of the data rises above the upper threshold or drops below the lower threshold, whereas at torques greater than 1250 Nm the measured torques are within the upper and lower thresholds.
The variation in the quantity of fuel injected for short on-times at no load results in a varying torque response of the engine and therefore varying engine speed responses. This behavior is not suitable for operation with an automated manual transmission as it results in poor shift quality (jerky/rough) and accelerated gear and/or clutch plate wear. Existing practices of injector calibration focus on characterizing a sample set of the fuel injectors on a test engine. The results of this characterization are stored in an engine controller, and the characterization is not adjusted over the lifetime of the fuel injector. Another calibration practice comprises coding each fuel injector and adjusting an average fuel injector characterization when the injector is installed on the engine. As the fuel injectors settle in the engine and age, any open loop characterization preset in the engine controller loses accuracy. There is a need to periodically calibrate fuel injectors in situ for the low fuelling portion of the plot in FIG. 1 in order to provide accurate fuel delivery and therefore torque and engine speed response required by the automated manual transmission.
With reference to FIG. 3, an automated manual transmission 20 can comprise a transmission control unit 30 that sends torque requests to an engine control unit 40 during shifting events. Engine control unit 40 responds to the torque requests and commands engine 50 to deliver the requested torque in order to provide efficient and smooth shifting of gears with minimal wear. A simplified example of a sequence for a clutchless-shift mode of operation is described next.
Transmission control unit 30 determines an upshift is required and sends a first torque request to engine control unit 40. While clutch 60 is still closed engine control unit 40 commands engine 50 to deliver the first requested torque. Typically, when the first requested torque is delivered to automated manual transmission 20 there is neither a propelling force on the driveline nor a retarding force due to engine braking, such that net torque is zero. The current gear is then released.
Transmission control unit 30 sends a second torque request to engine control unit 40 to deliver a second torque that slows down engine 50 in order to synchronize the speed. In other examples the transmission control unit 30 can request a number of intermittent torques between the first and second torque requests in order to encourage a smooth transition of engine speed, thereby providing quick and efficient upshifting. Automated manual transmission 20 engages the next gear when the second torque is delivered and the engine speed has synchronized.
In this one example involving an upshift at a particular operating condition the engine may be required to deliver many distinct torques under no load. Taking into consideration downshifting, double clutch operation and the multitude of gears, and types of shift events in heavy duty trucks and machinery it can be understood that there are many distinct, small torques required and precise engine speed responses to these torques for successful automated manual transmission operation.
U.S. Pat. No. 6,907,861, issued Jun. 21, 2005 to Asano et al. discloses an injection quantity control device for a diesel engine. A fuel injection control device of a diesel engine performs a learning injection during a no-injection period, in which a command injection quantity is zero. A difference between a variation in the engine rotation speed in the case where the learning injection is performed and a variation in the engine rotation speed in the case where the learning injection is not performed is calculated as a rotation speed increase. A torque proportional quantity is calculated by multiplying the rotation speed increase by the engine rotation speed at the time when the learning injection is performed. An injection correction value is calculated from a deviation between the actual injection quantity, which is estimated from the torque proportional quantity, and the command injection quantity. The command injection quantity is corrected based on the injection correction value. Asano et al. teach that the calibration comprises associating predetermined pulse-widths of the fuel injectors with an actual quantity of fuel injected, and correcting the predetermined pulse-widths to deliver the commanded injection quantity.
There is required a new and improved apparatus and method for calibrating the low fuelling behavior of fuel injectors installed in an engine and re-calibrating fuel on-times throughout the life cycle of the fuel injectors and the engine.